Macrophage Inhibitory Cytokine 1 (MIC-1) is a TGF-β superfamily protein first cloned on the basis of increased mRNA expression associated with macrophage activation (1). While MIC-1 is not expressed in resting macrophages, stimulation of macrophages by a number of biological mediators including TNF-α, IL-1 and M-CSF induce MIC-1 expression. Because of its induction by many pro-inflammatory cytokines, but failure of direct induction by lipopolysaccharide and interferon gamma, it has been hypothesised that MIC-1 may be an autocrine down-regulator of macrophage activation (1).
Although originally identified in activated macrophages, MIC-1 can be expressed in several tissues (2-5). Northern blots of human tissues indicate the presence of small amounts of MIC-1 mRNA in the kidney, pancreas and prostate, and large amounts in the placenta (2, 4). Large amounts of MIC-1 have also been detected by immunohistochemistry in biopsies of breast, colon and prostate cancer (5). However, MIC-1 is not detectable within normal epithelial cells of these organs (5). This, along with induction of MIC-1 expression by p53 and data suggesting that MIC-1 is able to induce apoptosis of some epithelial tumour cells lines (6-8), also suggests a role for MIC-1 in epithelial neoplasms.
Shortly after cloning MIC-1 cDNA, two allelic forms of this gene existing due to a single nucleotide polymorphism were identified. This polymorphism causes an alteration of a histidine (H) to an aspartic acid (D) residue at position 6 of the mature MIC-1 protein (9). The homozygous D allele is present in about 5% of normal individuals (10). Because the properties of these amino acids differ from each other, it was hypothesised that this may lead to alteration in some aspect of the function of the MIC-1 protein. In the course of undertaking epitope-mapping studies of a series of monoclonal and polyclonal anti-MIC-1 antibodies, it became apparent that one of these antibodies was able to discriminate between the H and D alleles (9), a property which allows the deduction of the genotype from the phenotype of the MIC-1 protein present in serum (10). These antibodies have also allowed the development of a sensitive immunoassay capable of quantifying MIC-1 in normal and pathological sera (2).
As MIC-1 is a product of activated macrophages, the present inventors considered that serum or plasma measurement of MIC-1 may be diagnostically and/or prognostically informative of cardiovascular disease, particularly atherosclerosis, since there is a wide body of data implicating activated macrophages in the pathogenesis of atherosclerosis and in the vascular occlusion that is often the ultimate end point of this process (11, 12). There is also strong epidemiological data linking measurement of inflammatory markers such as C-reactive protein (CRP) and IL-6 with the risk of vascular occlusive events (13, 14). The work presented hereinafter demonstrates that the typical increased inflammatory response present within atherosclerotic vessel walls is associated with increased secretion and release of MIC-1 and that the consequent increase in basal levels of MIC-1 is associated with an increased risk of future cardiovascular events.